Teratogens: Chemicals Which Cause Birth Defects
This page describes harmful substances, called teratogens, which a woman should avoid during pregnancy. These include certain chemicals, medications, social drugs, alcohol, smoking, and infections. Teratogens are substances or other factors that can cause congenital abnormalities, which are also called birth defects. Usually these abnormalities arise in the third to eighth weeks of pregnancy, when the major organ systems are forming. Examples of teratogens include certain chemicals, medications, and infections or other diseases in the mother.
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It is difficult to determine whether a particular chemical or medication causes congenital abnormalities. This is because many women take medications during pregnancy, and most studies have to rely on the mother's memory of what she took while she was pregnant. One notable exception is thalidomide , a medication used to treat morning sickness, which was found in the s to cause total or partial absence of the arms or legs in babies. Alcohol use is a well-known cause of congenital abnormalities during pregnancy.
Even moderate amounts of alcohol in pregnancy can cause developmental problems in the unborn baby. Abnormalities caused by alcohol in pregnancy include deformities of the face, arms, and legs, heart conditions, intellectual disability, and fetal growth restriction. However, these conditions are not very common.
More frequently, children born to women who drink heavily during pregnancy may have problems with thinking and remembering and behavioural issues.
The abnormalities and other problems caused by alcohol use in pregnancy are referred to as fetal alcohol spectrum disorder. Cigarette smoking is linked with fetal growth restriction and premature birth. Smoking may also cause problems with the development of the brain, cardiovascular system, and respiratory system.
This is true whether the mother smokes themselves or is exposed to second-hand smoke. Babies exposed to cigarette smoke during pregnancy may also be born with an increased startle reflex, tremor, or other problems. The effects of cigarette smoke on the unborn baby increase with how much the mother smokes, as well as the length of time that they have been smoking.
Exposure to marijuana during pregnancy may result in low birth weight, intracranial bleeding, jitteriness, low blood sugar, low levels of calcium in the blood, or an infection of the blood called sepsis. The use of marijuana in pregnancy can cause other problems in the baby such as poor feeding, irritability, and rapid breathing. Amphetamines, also called "speed," stimulate the central nervous system. Prenatal use of these drugs is associated with premature birth, low birth weight, or intracranial bleeding.
Teratogens: Things that Cause Birth Defects
The use of opioid drugs, such as heroin or methadone, during pregnancy can lead to fetal growth restriction, premature birth, and low birth weight. Cocaine use is known to cause numerous problems during pregnancy. These include miscarriage, fetal growth restriction, and problems with the development of the urinary system or genital tract. The use of cocaine can lead to microcephaly, where the brain is too small. Children of mothers who used cocaine during pregnancy are also more prone to developing neurobehavioural problems.
Cocaine use during pregnancy has been associated with a higher risk of a serious problem with the placenta, called placental abruption. Prenatal use of cocaine may also cause increased startling, jitteriness, and excessive sucking in the newborn baby. Many early Hebrews said that abnormal development resulted from the deformed person's association with the devil. Aristotle , who lived in Athens, Greece in the fourth century, B.
Aristotle and Hippocrates , a physician who practiced in Greece in the fifth century B.
The theory of maternal impressions persisted until the early s, despite evidence to the contrary by John Hunter , a surgeon in Scotland in the late eighteenth century. At the beginning of the 19th century, Johann Friedrich Meckel , the Younger, an anatomist from Halle, Germany, asserted that deviations from the normal developmental process caused malformations.
Meckel examined anatomical defects and their causes. Because he asserted that to understand abnormal development, one must first understand normal development, he documented his observations of normal embryological development of mammals in a sequence of forms. Meckel also categorized abnormal development into four basic types: reduced or absent body parts insufficient generative energy , enlarged or multiple body parts excessive energy , aberration of form and of position, and hermaphroditism, which included deformities such as ambiguous genitalia.
Following Meckel, scientists in the nineteenth century began experimental studies to detect teratogens. Etienne Geoffroy Saint-Hilaire in Paris, France, experimented on chick eggs by subjecting them to pricking, inversion, jarring, and abnormally high or low temperatures to study the resulting malformations; he believed that certain manipulations could invoke specific deformations.
Although deformities materialized, Saint-Hilaire didn't identify their exact causes. Other scientists following Saint-Hilaire also experimented with teratogens, notably Camille Dareste in France, who successfully produced abnormalities in chick embryos during twenty-two years of experiments until his death in Scientists in the twentieth century classified teratogens into four categories, physical, chemical, or infectious agents and maternal conditions.
Physical agents include ionizing radiation or other agents that contribute to hyperthermia, or elevated body temperature. Ionizing radiation is radiation composed of particles, X-rays, or gamma rays that carry adequate energy to free an electron from an atom or molecule, resulting in electrically charged ions in matter.
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In the s reports surfaced of abnormalities in the children of women who were X-rayed while pregnant. The common anomalies were small head circumference, or microcephaly , and small eyes, or microphthalmia. Douglas P. Murphy, from the University of Pennsylvania in Philadelphia, Pennsylvania, surveyed gynecologists and radiologists across the US between and , and found that of the seventy-four children reported to have been exposed to radiation in utero , twenty-five were malformed. Agents that cause hyperthermia are also physical teratogens. These could be saunas, hot tubs, or infections that raise a pregnant woman's body temperature to degrees Fahrenheit or higher.
Scientists have shown that hyperthermia-causing agents acted as teratogens in both animals and humans. Experiments on animals, such as guinea pigs, hamsters, rats, mice, rabbits, sheep , pigs, and monkeys from the early s to the early s demonstrated that hyperthermia causes central nervous system malformations, microcephaly , abdominal wall defects, defects of the eye and palate, and limb reduction defects. David L. Cockroft and Denis Alan Trevor New in the UK reported in the s that heating explanted rat embryos caused microcephaly , enlarged hearts, and skeletal deformities.
In the s, scientists used other agents to cause hyperthermia in pregnant females, such as ultrasound and electromagnetic radiation , to test the agent's capacity to cause birth defects in offspring. In humans , hyperthermia is associated with neural tube defects, spontaneous abortions, and various cardiovascular abnormalities. Physicians and scientists gathered evidence in the early s which supported their theories that there was an association between the high fevers of pregnant women and congenital abnormalities, such as cardiac defects, abdominal wall defects, or a disruption of the enervation of the large intestine called Hirschsprung disease, in their offspring.
Metabolic conditions affecting pregnant females such as malnutrition, diabetes, and thyroid disorders are a second category of teratogens.
Medical Genetics: Teratogens - Health Encyclopedia - University of Rochester Medical Center
Metabolic conditions are abnormalities in the chemical process of producing energy from food, and thereby affect the development and function of the body. If a pregnant woman is malnourished, then her fetus likely lacks the nutrients essential for its development. In the case of diabetes, low blood sugar, or hypoglycemia, may cause fetal malformations. Hypoglycemia interferes with some proteins in the developing fetal heart by increasing the expression of proteins which are regulated by glucose.
Excessive blood sugar, also seen with diabetes, may cause neural tube defects, or birth defects of the brain and spinal cord, and may also induce the release of free radicals, or damaged cells that are missing an essential molecule, which disrupt fetal development. In the s, The Society of Obstetricians and Gynaecologists of Canada reviewed reports from to on the risk of congenital anomalies in pregnancies caused by pre-existing or gestational diabetes. They found that the risk of major malformations increases from four to ten percent in infants of diabetic mothers.
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This statistic was two to three times higher than that in general populations. Thyroid disorders include disorders in which the thyroid gland malfunctions, thereby producing abnormal amounts of the thyroid hormones , thyroxine and triiodothyronine, which regulate metabolism. Thyroid disorders can cause a number of teratogenic effects to a developing fetus , as well as adverse effects on pregnancy such as miscarriage , premature separation of the placenta from the uterine wall placental abruption , preterm labor, and lower IQ scores in the children.
In the s, pediatrician Josef Warkany and colleagues in Cincinnati, Ohio raised female rats on a diet high in substances which interfere with thyroid function or goitrogens. As a result of this diet, the pregnant females developed enlarged thyroid glands and their offspring had skeletal malformations such as abnormally short jaw bones and tails, shortened or absent lower leg bones, and fusion of the ribs. Infections such as those caused by rubella virus, herpes simplex virus, and syphilis, are a third kind of teratogen.
In , ophthalmologist Norman McAlister Gregg at the Royal Alexandra Hospital for Children in Sydney, Australia witnessed cataracts in seventy-eight children whose mothers were infected with the rubella virus during either the first or second months of pregnancy. The connection Gregg made between the virus and congenital malformations contributed to one of the first discoveries of a teratogen that wasn't a manufactured chemical.
In addition to rubella, herpes simplex virus, and cytomegalovirus—one of the herpes viruses that passes through direct contact with body fluids, congenital abnormalities can be caused by infections of Toxoplasma gondii , a parasite often obtained by eating contaminated meat, drinking contaminated water, or coming into contact with infected cat feces, and Treponema pallidum , the bacterium that causes syphilis.
The fourth kind of teratogen includes drugs and chemicals the pregnant female ingests such as alcohol, cocaine, thalidomide, Agent Orange, and vitamin A and its derivatives, called retionids. In , Fred Hale at the Texas Agricultural Experiment Station in College Station, Texas, fed pregnant female pigs a diet severely deficient in vitamin A and found that the offspring had various congenital malformations such as anophthalmia, which is the absence of one or both eyes, and cleft palate. Over the next four years, Hale experimented with pigs and vitamin A deficiency and found other defects such as cleft lip and malformed hind legs.
Hale's experiments established that an absence or deficiency of a nutrient could produce severe congenital malformations in mammalian embryos. In , pediatrician Sidney Q. Cohlan in New York City, New York, reported that large doses of vitamin A caused congenital malformations of the central nervous system and other systems in rats. That announcement led to decades of experimentation with vitamin A and its derivatives, retinoids.